The deposition of organic contaminants (i.e., pitch and stickies) on equipment surfaces in the papermaking process is well known to be detrimental to both product quality and the efficiency of the papermaking process. Some contaminating components occur naturally in wood and are released during various pulping and papermaking processes. Two specific manifestations of this problem are referred to as pitch (primarily natural resins) and stickies (adhesives or coatings from recycled paper). Pitch and stickies have the potential to cause problems with deposition, quality, and efficiency in the papermaking process.
The term “pitch” herein refers to deposits composed of organic constituents that may originate from natural wood resins, their salts, as well as coating binders, sizing agents, and defoaming chemicals which may be found in the pulp. In addition, pitch frequently contains inorganic components, such as calcium carbonate, talc, clays, titanium and related materials.
“Stickies” is a term that has been increasingly used to describe deposits that occur in systems using recycled fibers. These deposits often contain the same materials found in “pitch” and also may contain adhesives, hot melts, waxes, and inks.
When organic contaminants, such as pitch and stickies, deposit on surfaces in papermaking, quality and operating efficiency of a pulp or paper mill may be impacted or reduced. Organic contaminants can deposit on process equipment in papermaking systems resulting in operational difficulties in the systems. When organic contaminants deposit on consistency regulators and other instrument probes, these components can be rendered unreliable or useless. Deposits on screens can reduce throughput and upset operation of the system. This deposition can occur not only on metal surfaces in the system, but also on plastic and synthetic surfaces, such as machine wires, felts, foils, Uhle boxes and head box components.
Historically, the subsets of the organic deposit problems, “pitch” and “stickies”, have manifested themselves separately, and differently, and have been treated separately and distinctly. From a physical standpoint, “pitch” deposits usually have formed from microscopic particles of adhesive material (natural or man-made) in the stock that accumulate on papermaking or pulping equipment. These deposits readily can be found on stock chest walls, paper machine foils, Uhle boxes, paper machine wires, wet press felts, dryer felts, dryer cans, and calendar stacks. The difficulties related to these deposits include direct interference with the efficiency of the contaminated surface, leading to reduced production, as well as holes, dirt, and other sheet defects that reduce the quality and usefulness of the paper for operations that follow, like coating, converting or printing.
From a physical standpoint, “stickies” usually have been particles of visible or nearly visible size in the stock that originate from recycled fiber. These deposits tend to accumulate on many of the same surfaces on which “pitch” can be found and cause many of the same difficulties that “pitch” can cause. The most severe “stickies” related deposits, however, tend to be found on papermaking machine wires, wet felts, dryer felts and dryer cans.
Methods of preventing the build-up of deposits on the pulp and paper mill equipment and surfaces are of great importance to the industry The paper machines could be shut down for cleaning, but ceasing operation for cleaning is undesirable because of the consequential loss of productivity. Deposits also can result in poor product quality, which occurs when deposits break off and become incorporated in the sheet. Preventing deposition thus is greatly preferred where it can be practiced effectively.
In the past, stickies deposits and pitch deposits more typically have manifested themselves in different systems. This was true because mills usually used only virgin fiber or only recycled fiber, and did not mix these furnish slurries together. Often very different treatment chemicals and strategies were used to control these separate problems.
Current trends are for increased mandatory use of recycled fiber in all papermaking systems. This is resulting in a co-occurrence of stickies and pitch problems in a given mill. In addition, with increased recycling of fibers, the trend towards “microstickies”, defined as stickies with a diameter less than 150 μm, is a growing concern. Microstickies, because of their small size and large surface area, present a greater tendency to deposit and/or agglomerate.
In order to establish the best means to treat or prevent such problems, it is desired to have a method that can predict likelihood of particulate contaminant deposition, and that can quantify its effects as well as the effects of the various potential treatments that may be implemented.
In order to determine the contaminant content in a pulp, methods that measure the deposition of organic contaminants on a specific substrate by gravimetric analysis have been used. U.S. Pat. No. 6,090,905 teaches a method wherein the weight differential of packaging foam, placed in stainless steel baffles, before and after exposure to pulp slurry is utilized to estimate the content of deposited stickies. European Pat. No. EP 0 922 475A1 discloses a device that accumulates deposit under a shear field brought about by a rotating disc.
A variation to the gravimetric methods to quantify deposition is the use of sensors that respond to the weight of the deposit. U.S. Pat. No. 5,646,338 teaches an apparatus that relates the amount of lateral deflection about a pivot of a cantilever probe to the build-up of deposit on the projection portion of said probe. U.S. Pat. Appl. Pub. No. 2006/0281191 A1 teaches the use of a Quartz crystal microbalance whose vibration frequency and amplitude is affected by the formation of deposits on the exposed side of the crystal.
A drawback with gravimetric methods of measuring deposition quantity is that the actual measurement has a high potential for variability because of the small weight of deposit on the substrate. Gravimetric methods also typically quantify the total amount of contaminant which may not correspond to the amount that is deposited. Sensors can also be problematic when they are introduced into high shear environments or where there are mechanical vibrations in the fluid. Consequently, these methods may not be able to characterize the efficacy of a deposition treatment program.
Methods to specifically quantify stickies in pulp are summarized by J. Dyer, “A Summary of Stickies Quantification Methods,” Progress in Paper Recycling, pp. 44-51 (1997, August). These methods include image analysis techniques, such as that employed with the Pulmac MasterScreen (Pulmac International, Montpelier, Vt., USA) low consistency screening device designed to mechanically separate fibers from contaminants including stickies and shives. R. Blanco, et al., “New System to Predict Deposits due to DCM Destabilization in Paper Mills,” Pulp & Paper Canada, 101(9), pp. 40-43 (2000), discloses a variation of the equipment disclosed in European Pat. No. EP 0 922 475A1 employing image analysis techniques. K. Cathie, et al., “Understanding the Fundamental Factors Influencing Stickies Formation and Deposition,” Pulp & Paper Canada, 93(12) (1992), pp. 157-160, discloses a method wherein stickies deposition on forming wires is quantified by image analysis.
Some image analysis techniques do discriminate between different types of contaminants to quantify those which specifically results in deposition. However, they typically are not capable of quantifying microstickies. Improved methods and apparatus for collecting particulate contaminants, diagnosing stickies and pitch formation and evaluating effectiveness of prevention treatments continue to be sought.